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Located in the western circum-Pacific seismic zone, Taiwan is an ideal area for recording tsunamis that originate in the tectonically and volcanically active systems of the Ryukyu Arc, Okinawa Trough, and Luzon Arc. However, because of Taiwan's short historical record, there is only credible evidence for one disastrous tsunami, which occurred in northern Taiwan in AD 1867. To better understand the western Pacific tsunamis, an Upper Holocene deposit of a coastal plain inundated by the AD 1867 tsunami was investigated to search for geological evidence of tsunamis. Based on facies characteristics and sediment composition, we identified two sharp-based, decimetre-thick marine quartzose sandy layers that fine upward and landward, intercalated with floodplain marsh sand and mud. The fluvium are rusty red, rich in volcanic clasts and Fe, and comparable to modern riverine sediments of local andesitic provenance. Extending 0.8 km inland, the marine quartzose sandy layers are light-coloured and rich in quartz and Si, have high Ca/Ti values, and are comparable to the modern beach–dune sediments of regional siliciclastic provenance. Radiocarbon dating indicates that the marine sandy layers are correlated with the AD 1867 tsunami and an older event at the boundary between the 17th and 18th centuries. Both events were probably associated with reported earthquakes involving normal faulting. In addition, the distribution patterns of the two marine sandy layers are similar, suggesting that the inundation distances, run-ups, and flow heights of the two events are comparable. This indicates that the disastrous tsunamis were frequent in the area between northern Taiwan and the Okinawa Trough of the western circum-Pacific seismic zone. [ABSTRACT FROM AUTHOR]

Encircled by arcs and trenches, the western Pacific region of the Philippine Sea is exposed to trans-oceanic tsunamis and pumice rafts. However, shortage of extended archive has limited our knowledge of tsunami recurrence, pumice sources, and drifting routes. This study identifies a backshore colluvium with three new pumice gravel layers in northern Taiwan, stratigraphically below four previously reported pumice-bearing paleotsunami deposits from the last millennium. The new layers are attributed to tsunamis, based on foreshore-derived sediments, facies characteristics reflecting suspension-rich turbulent flow process, supratidal locations exceeding typhoon surge heights, and mismatched super typhoon occurrences. The tsunami record therefore extends in duration from 1.0 to 2.4 ka with an increase in event number from four to seven, which unfolds new time intervals, stretches the recurrence from 100–400 to 100–800 years, and indicates complicated tsunami occurrences and sources in this region. Simultaneously, a stranded pumice archive is established by six of the seven paleotsunami deposits with abundant pumices rounded by grain-to-grain abrasion during drifting. The pumices consist of black, shoshonitic trachyte–trachyandesite grains and white, low-K dacite–rhyolite grains, indicating diverse sources and drifting routes. The unique alkali and adjoining subalkaline volcano provinces of the Izu-Bonin-Mariana Arc might have successively generated pumice rafts since 2.4 ka that embarked the Kuroshio counter and branch currents for northern Taiwan in a ∼2000-km-long course across the Philippine Sea. White rhyolite pumice domination is also present and comparable in composition to the southern Ryukyu pumice, indicating a ∼200 km-long northwest-bound route. • A prolonged record of western Pacific tsunamis and pumice rafts is established in northern Taiwan for the first time. • Seven pumice-bearing tsunami deposits from 2.4 ka recurred at a 100–800-year interval. • Compositional ties to the Izu-Bonin-Mariana Arc pumice rafts across the Philippine Sea are prominent. • Pumice comparable to those from the adjacent southern Ryukyu Arc occasionally dominated. [ABSTRACT FROM AUTHOR]

Widely known for its hazardous history, Taiwan holds valuable records of NW Pacific tsunamis and typhoons. Despite the similarity between tsunami and typhoon deposits, historical accounts enabled the identification of the 1867 tsunami deposit on the northern coast through matching with the record-setting inundation. However, the identification of prehistoric tsunami deposits has been hindered by severe sediment reworking and post-depositional changes produced by the typhoon-prone, tropical, and humid climate. In this study, we investigated the upper Holocene facies and their sediment sources and radiocarbon ages on distal floodplains and rocky backshores of the northern coast, where sediment reworking is less intense. Four decimetre-thick gravelly layers were identified, intercalated with fluvial mud and colluvium. The gravelly layers are composed of beach-derived quartz-rich sand and rounded gravels of the sedimentary basement rocks, longshore-drift andesite and stranded pumice. The remarkable layer thickness, gravel size, and quartz richness were determined to have endured post-depositional changes. The event layers are sharp-based, matrix-supported, and poorly sorted and have recorded scouring, sediment entrainment and rapid fallout of suspension-rich turbulent flows that probably originated with tsunamis. The event layers occur 0.5–0.8 km inland and 3–11.5 m above sea level, matching the 1867 tsunami inundation and wave height, respectively. Radiocarbon dating tied the event layers to the 1867 tsunami and three prehistoric events that occurred in the periods: 1545 to 1810, 1293 to 1414 and 1090 to 1235, representing a recurrence interval of 83–436 years over the last millennium. • A new geological tsunami record of Taiwan of the last millennium is established. • A historical and three prehistoric events occurred at an interval of 83–436 years. • Event deposits have endured long-term reworking and post-depositional changes. • Reworking is minimised on distal floodplain and gentle rocky backshore. • Decimetre-thick layers of gravel- and quartz-rich contents bear high endurability. [ABSTRACT FROM AUTHOR]